WOW...you just opened a big can of worms!!! They do work together. After all, velocity is part of the physics equation to get energy. There's two camps on this subject. Ones says that speed (with sufficient energy) kills the best. The other says that energy (with sufficient velocity) kills the best. See the problem here? THEY BOTH KILL!!!! Such arguements between the two camps are almost always ended "better" or "best"...but dead is dead.

The pratical difference is that the high velocity rounds will ruin more meat...when you dress a deer shot with a 7mm mag (which leans towards the velocity preference) and one shot with a 45-70 (which leans towards energy) you will see the difference. The 7mm will alot of ruined meat around the entry and exit hole..the 45-70 leaves you the option to "eat right up to the whole".

They both kill, sometimes by different methods...but they both kill.

I should add that the advantage of the faster cartridges (usually) is that they shoot flatter and you don't have to compensate for yardage...as much. While another advantage of the slower cartridges (which usually shoot heavier bullets) is that you don't have to worry about bullets 'failing' and if it its a shoulder...that shoulder, and that animial, is destroyed. Whereas the lighter/faster bullet *might* hit the shoulder bone and fall apart. Leaving you with a tough tracking assignment.